Skin barriers resisting against infiltration of external harmful materials into the skin are largely present on the strateum corneum of the skin epidermis. Such a strateum corneum functions to protect the human body from physical damages caused by external factors and from chemicals, and interrupts evaporation of moisture in the human body to prevent skin dryness. Active studies have been conducted about lipid ingredients in the structure of the strateum corneum. Elias and coworkers have reported that the lamellar structure formed of the lipid ingredients in the stratum corneum is the origin of such a skin barrier function (J. Invest. Dermatol. 69: 535-546, 1977). In addition, although there is a difference depending on the experimental method and the position of a skin tissue, it is known that intercellular lipids substantially include about 50% of ceramides, 20-25% of cholesterol, 20-25% of free fatty acids, about 10% of cholesterol ester, 1-2% of cholesterol sulfate and a small amount of phospholipids.
To apply such a skin barrier function to cosmetics, many studies have been conducted to simulate the skin barrier function and various formulations, such as a liquid crystal emulsion, have been developed. The main purpose of such formulations is stable incorporation of intercellular lipids that are main ingredients providing such a skin barrier function, and many studies have been conducted about ceramides, cholesterols and phospholipids. Particularly, since it was known that ceramides and cholesterols are not only main ingredients of intercellular lipids that are main ingredients of skin barriers but also strong moisturizing ingredients, stabilization thereof has been studied intensively. However, when using them in conventional emulsion formulations, they undergo gelling with time. Thus, in most cosmetic formulations, they are used in a very small amount, or are subjected to a separate pretreatment process using a high-pressure emulsification system, such as a microfluidizer, to form a stabilized material before they are applied to cosmetics. However, such high-pressure emulsification requires a special system and complicated process and shows undesirably low cost-efficiency. To overcome the problems, many studies have been conducted to solubilize ceramides or to convert ceramides into nanoliposomes through high-pressure emulsification.
For example, Korean Patent Publication No. 10-2005-0026778 discloses ‘Cosmetic Composition for Alleviating Skin Irritation Including Nanoliposomes of Intercellular Lipid Ingredients’ and Korean Patent Publication No. 10-2011-0076068 discloses ‘Vesicles Including Self-Emulsifiable Nanoliposomes/Multilayer Liquid Crystals, and Preparation and Use Thereof’. In both patent documents, nanoliposomes are obtained by using high-pressure emulsification in order to stabilize intercellular lipid ingredients including ceramides, cholesterols and phospholipids. However, the methods according to both patent documents use a special system (i.e., high-pressure emulsification system), and thus require additional cost and time undesirably. In addition, Korean Patent Publication No. 10-2010-0100202 discloses a stable solubilized formulation containing micelles of intercellular lipid ingredients having a size of 1-200 nm and the solubilized formulation does not require high-pressure emulsification. However, the obtained stable solubilized formulation is limited to a formulation having a selected size of approximately 1-200 nm. In addition, according to the studies of the present inventors, it has been found that such a formulation causes a problem related with stability, including precipitation of intercellular lipid ingredients during the storage at 45° C. and 50° C., and during the thawing after the refrigeration at −20° C. It is known that a solubilized formulation obtained by dissolving sparingly soluble/insoluble materials, such as perfume or oil, having hydrophobic property and hardly water-soluble property into water by using a material (solubilizing agent) having amphiphilic property (i.e., both hydrophilicity and hydrophobicity) has one phase having a thermodynamically stable and uniform spherical structure. Since such a solubilized formulation is thermodynamically stable, it is very stable regardless of mixing methods and orders, when the conditions, such as temperature or composition, are compatible. However, because highly hydrophobic materials, such as ceramides or cholesterols, to be solubilized in the above-mentioned manner allow solubilization only in the inner part of the solubilizing agent (i.e., in the core spaces), there are not sufficient spaces for solubilization. Therefore, highly hydrophobic materials to be solubilized require a significantly larger amount of solubilizing agent as compared to the solubilization process of perfume or oil. In this case, some problems, including a sticky feeling of use and irritation to the human body, may occur undesirably.
Under these circumstances, the inventors of the present disclosure conducted intensive studies to overcome the above-mentioned problems. After the studies, we have found that it is necessary to minimize mass transfer through the interface in order to improve the stability of a nanoemulsion containing a large amount of intercellular lipid ingredients. Based on this, we have found a novel method for preparing a nanoemulsion containing a large amount of intercellular lipid ingredients. When a nanoemulsion is obtained by the novel method, it is possible to reduce processing time and cost since a non-ionic surfactant is used without a high-pressure emulsification process, and the obtained nanoemulsion is more stable against a change in temperature as compared to the other solubilized formulations. The present disclosure is based on this finding.